The basic optic function of the eye is to form an image of whatever object being looked at (fixated on) on the retina. This image formation is done by the combination of the cornea and the lens which behaves like a convex lens. The focus length of this lens is adjustable by the muscles attached to it according to the distance of the object so that its image is always formed on the retina in normal situation.
The simple relationship between the focal length f of the lens and the
distances of the object and its image is
In the human eye, as the image always needs to be formed
on the retina, di is fixed to be the distance between the lens and the retina,
which is about 0.017 meters (17 mm), and the focal length of the lens need to be
adjuected as a function of the distance of the object:
The power of a lens to bend the incoming light is measured by diopter, defined as the reciprocal of the focal length in meter. To see objects of different distances, the power of the lens needs to be changed accordingly. When the object is far away ( ), its image is formed at the distance equal to the focal length ( di=f=0.017 mm); however, when the object is close, its image will be formed farther away from the lens (di>f). For the image to still be formed on the retina, the lens needs to change its shape to increase its power (shorten f). As one can see in the plot below (showing f and 1/f as functions of object distance do, given di=17 mm), major adjustment of the lens is needed only when viewing objects within the range of about half a meter. For example, the focal length needs to be at 0.0145 meters for an object at 0.4 meters, or 0.0163 meters for an object at 0.4 meters. To view any object farther away than 0.5 meters, the lens needs to be pretty relaxed so that f is very close to 0.017 meters.